1. Field of the Invention
The present invention relates to a hydraulic control apparatus for use with an automatic transmission of a vehicle and particularly to improvements of a fail-safe technique that enables a vehicle to run, i.e., “escape” when transmission-related solenoid valves fall in a so-called “off-fail” state.
2. Discussion of Related Art
There is known a hydraulic control apparatus for use with an automatic transmission of a vehicle. The automatic transmission includes a plurality of hydraulically operated, frictional coupling devices, and establishes each one of a plurality of speed steps having respective different speed ratios, when the frictional coupling devices are selectively engaged or released (disengaged). The hydraulic control apparatus includes a plurality of transmission-related solenoid valves that are provided in association with the frictional coupling devices, respectively, and control respective hydraulic pressures supplied to the frictional coupling devices. Japanese Patent Application Publication No. 9-303545 discloses an example of the hydraulic control apparatus, and proposes to provide, between the transmission-related solenoid valves and the frictional coupling devices, a fail-safe valve that supplies, when the transmission-related solenoid valves fall in the “off-fail” state in which the solenoid valves cannot output respective hydraulic pressures, a hydraulic pressure to a pre-selected one of the frictional coupling devices so as to establish a pre-selected one of the speed steps, and thereby enables a vehicle to run, i.e., “escape”.
However, in the above-indicated conventional hydraulic control apparatus, the fail-safe valve is provided between the transmission-related solenoid valves and the frictional coupling devices. Therefore, when the solenoid valves are normal, the above-indicated pre-selected frictional coupling device receives the hydraulic pressure via the fail-safe valve. This leads to increasing a length of a fluid (oil) channel and thereby lowering a responsiveness, and a control accuracy, of each of the solenoid valves with respect to its hydraulic control.